Generally, in a seal unit used in a hydraulic apparatus having a piston and a cylinder, a seal ring is fitted onto the inner circumferential surface of the cylinder. The seal ring is provided with an annular lip which is wound with an elastic clamping member, such as a coil spring, so as to be pressed onto a shaft of a piston. A high fluid pressure bears radially and axially on the annular lip on the coil lubricant side of the seal. In an ordinary seal unit in which a seal ring is constituted only by elastic matter such as rubber or the like, the annular lip is often deformed or damaged when it is pressed by the piston shaft. As a result, the seal ring cannot perform its sealing function satisfactorily.
In order to prevent such a deformation from occurring to the annular lip, it has been proposed to fit a backup ring of a rigid material, such as metal, plastic, or the like, in an inner circumferential surface of the seal ring and in the air-side back surface of the annular lip of the seal ring. Furthermore, since it is necessary to cause the piston shaft having an outer diameter larger than the inner diameter or the seal ring, to pass through the latter in assembling the apparatus, there has been proposed a backup ring that is cut in a circumferential portion so as to make the ring extensible in the radial direction. This is disclosed in U.S. Pat. No. 4,468,041.
There has also been proposed a backup ring in which the ring-like body thereof is cut in such a manner that the opposite cut end portions of the body fit together through a concavo-convex engagement so as not to cause a relative discrepancy in the radial direction at the opposite cut end portions. This is disclosed in Japanese Utility Model Publication No. 65464/1983.
In any of the proposals described above, however, the fact that part of the circumference of the backup ring has been cut off causes radial or axial misalignment between the opposite cut ends thereof. As a result, the backup ring often slips off when it is attached to a seal ring or when the seal ring with such a backup ring mounted onto a piston. The backup ring also often damages the seal ring to which it is attached by biting into the seal ring because of misalignment of the opposite cut end surfaces of the backup ring when the seal ring is in use with the backup ring.
Sometimes, the backup ring causes deterioration of the sealing function because of abrasion caused by abnormal contact with a piston. Moreover, it is difficult to form concavo-convex portions at the opposite cut ends of the backup ring and it is impossible to prevent damage from occurring in the seal ring due to the axial misalignment between the opposite cut end surfaces.